1. Field of the Invention
The present invention relates to a method of detecting an angle of optical rotation, and a detecting apparatus and a detector cell for realizing the method.
The measurement of the angle of rotation is utilized in a detector such as a detector of a liquid chromatograph, and realizes, for example, a qualification and determination of optically active substances by measuring an angle of rotation caused by the optically active substances contained in eluent discharged from a column of a liquid chromatograph.
2. Description of the Related Art
Recently, the creation of an optical resolution column has enabled analyses of optically active substances to be widely carried out by using a liquid chromatograph. Currently, some types of apparatuses for measuring the angle of rotation are also used as a means for detecting the optically active substances, in the liquid chromatograph.
One type of such apparatus comprises a Faraday cell for irradiating a sample solution with a linearly polarized light beam having a direction of vibration modulated by an alternative signal. The intensity of transmitted light is analyzed in an analyzer having a direction orthogonal to a mean direction of vibration of the irradiating polarized light and a photo-electric element. When the sample solution does not contain an optically active substance, the intensity gives an alternative current signal which does not contain a direct current component, and when the sample solution contains optically active substances, the intensity gives an alternative current signal which contains a direct current component corresponding to an angle of rotation by the sample solution. Accordingly, an output signal of the photo-electric element is synchronously demodulated with a modulation signal of the Faraday cell, to obtain a signal corresponding to the angle of rotation by the sample solution.
This type of detection apparatus is disclosed in Japanese Unexamined Patent Publication No. 55-103434, although the apparatus is not disclosed in the publication as a detection apparatus for liquid chromatograph but as an azimuthal polarimeter.
On the other hand, in a detection apparatus disclosed in Japanese Unexamined Patent Publication No. 61-83924, a sample solution is irradiated with a non-modulated linearly polarized light beam, and the intensity of transmitted light is analyzed in an analyzer having two directions orthogonal to each other and not parallel to that of the irradiating polarized light and two respective photo-electric elements. An angle of rotation by the sample solution is calculated from the two intensity values, using a certain formula.
However, since these detection apparatuses are based on a measurement principle of an azimuthal polarimeter wherein a sample solution filling a detector cell is stationary, where a detection is carried out while the sample solution flows through a flow cell at a constant flow velocity, a large noise signal due to a time-dependency of concentration of the sample appears in the output signal. If these azimuthal polarimeters are used as a detector of a liquid chromatograph, peaks in chromatogram are distorted and a large noise signal appears even while an optically non-active substance is passing through the detector cell, and therefore, it is difficult to use them as a detector of optical active substances in a liquid chromatograph, etc.